This paper presents results from an examination of teacher flows within the San Diego school system, but should be relevant to school personnel systems in large cities throughout the U.S. The study was undertaken because: (a) traditional demand-supply analyses have been unable to solve many of the puzzles associated with the general topic of labor mobility, and (b) the implications of teacher mobility patterns on student achievement must be examined. The focus was on mobility among schools within the system, movement into and out of the system, and the effect of these flows on the allocation of teachers among different types of schools. In general it was found that incoming teachers are most likely to move to another school. Highly experienced teachers are least likely to move, presumably because they have already found an assignment to their liking. Although teachers do leave the system for more appealing opportunities outside it, few leave because they are dissatisfied with their particular assignment--they tend to seek a more satisfactory position within the system. Beginning teachers are usually placed in low socioeconomic status (SES) schools and tend to move to higher SES schools, so that the oldest, most experienced, most educated teachers are found in these schools. As for how teacher mobility affects student achievement, it appears that new teachers are assigned to schools where students do poorly on achievement tests, and these teachers soon move on to middle-class schools attended by superior test takers. (PB)